Private variables and methods

A private variable or private method is a variable that is not visible outside the methods of the class in which it is defined. Private variables and methods are useful for two reasons:

  1. they increase security and reliability by selectively denying access to important parts of an object’s implementation

  2. they prevent naming conflicts that can arise from the use of inheritance.

A class can define a private variable and inherit it from a class that defines a private variable with the same name. Private variables make code easier to read because they explicitly state what is only used internally in a class. Everything else is the interface of the class.

Most languages that define private variables do so by using the keyword private or similar. The convention in Python is simpler and also makes it easier to see immediately what is private and what is not. Any method or instance variable whose name begins with a double underscore (__) but does not end is private; anything else is not.

As an example, consider the following class definition:

>>> class MyClass:
...     def __init__(self):
...         self.x = 1
...         self.__y = 2
...     def print_y(self):
...         print(self.__y)
...
>>> m = MyClass()
>>> print(m.x)
1
>>> print(m.__y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyClass' object has no attribute '__y'

The print_y method is not private, and since it is in the MyClass class, it can access and output __y:

>>> m.print_y()
2

Warning

The mechanism used to ensure privacy falsifies the name of private variables and private methods when the code is compiled into bytecode. Specifically, this means that _classname is prefixed to the variable name:

>>> dir(m)
['_MyClass__y', '__class__', …]

So this is only to prevent accidental access.

Checks

  • Modify the code of the Triangle class to make the dimension variables private. What restriction will this change impose on the use of the class?

  • Update the dimensions of the Triangle class so that they are properties with getters and setters that do not allow negative values.